There are a number of conventional methods of converting linear motion into rotary motion to actuate mechanical devices. Among these methods are rack and pinion gears, rotary solenoids, eccentric axles and crank arms, and pulley systems including differential pulleys.
The prior art differential pulleys have been employed to achieve high mechanical advantage by converting a large force acting through a small distance into a relatively large rotary motion. Differential pulleys of this type have been used in a variety of energy conversion devices. Other uses include windlasses to produce continuous motion or to lift weights. A typically configuration is a differential pulley tackle in which an endless wire or cable passes through a movable lower pulley, which carries a load, and two coaxial, conjointly rotating, upper pulleys having different diameters.
The differential pulley configuration is unique in that mechanical advantage is greatest when the pulley diameters are nearly the same. The wire or cable which unwinds from the large pulley is nearly all taken up on the small pulley so that there is a low rate of change in length of the wire or cable path relative to rotation of the pulley.